This invention relates generally to turbocharged diesel engines that propel motor vehicles and are equipped with a variable valve actuation mechanism that can vary the timing of engine valve opening, and in particular to a control strategy for increasing engine torque without undesirable consequences on tailpipe emissions, such as diesel engine exhaust smoke, especially at lower engine speeds where turbocharger boost is relatively low.
A turbocharger is one type of device that is used to supercharge an internal combustion engine. A diesel engine that is supercharged by a turbocharger is sometimes referred to as a turbocharged diesel. A turbocharger comprises a turbine that is powered by engine exhaust gas and coupled by a shaft to operate a compressor that boosts pressure in the engine air intake system downstream of the compressor. Boost is controlled by controlling turbine operation.
A strategy for controlling turbine operation needs to consider the particular type of turbocharger. One type of turbocharger has a variable geometry, or variable nozzle, that is capable of changing the manner in which exhaust gas that flows through the turbocharger interacts with the turbine. Movable vanes are selectively positioned to control the nature of exhaust gas interaction with the turbine, and hence control boost. The turbocharger includes an electromechanical actuator for interfacing an electric control with the movable vanes. That actuator comprises a solenoid for setting vane position according to a control signal from the electric control. The control signal is developed according to a desired control strategy.
A waste-gate type turbocharger controls the proportion of exhaust gas that is allowed to interact with the turbine by controlling the extent to which a waste gate valve that diverts exhaust gas from the turbine is allowed to open. The waste gate valve may be operated by an electric actuator to which a control signal is applied.
It is believed fair to say that a turbocharger is generally considered to be a device for improving engine performance. A turbocharger is typically designed for higher engine speeds, because the amount of engine exhaust that is available to act on a turbine of a turbocharger at low engine speeds is usually insufficient for the turbocharger compressor to develop sufficient boost to render it effective in contributing to improved performance at those low speeds.
It has been discovered however that certain turbocharged diesel engines, especially engines that have a variable valve actuation mechanism that can vary engine valve timing, can develop increased low speed torque without undesirable consequences on tailpipe emissions, such as smoke in the engine exhaust. This improvement is achieved by certain conjunctive control of: 1) time at which the engine exhaust valves open during an engine operating cycle and 2) engine fueling. In general, the conjunctive control comprises retarding, i.e. delaying, the opening of the exhaust valves while increasing the fueling to maintain a desired air-fuel ratio in the combustion chambers. The improvement can provide a significant increase in engine torque during low speed operation of the engine without significant adverse effect on tailpipe emissions.
The process of exhausting products of combustion from a combustion chamber of a diesel engine may be considered to comprise two phases: 1) a blow-down phase where the exhaust gas pressure is large enough to induce exhaust gas flow through an open exhaust valve; and 2) a pump-out phase where the moving engine mechanism is reducing the swept volume of the combustion space to an extent that forces exhaust gases out through the open exhaust valve. The blow-down phase will commence immediately upon opening the exhaust valve while the pump-out phase will occur later. For example, if the exhaust valve for an engine cylinder is opened as a piston is completing a power downstroke within the cylinder in advance of the piston""s arrival at bottom dead center (BDC), the blow-down phase will commence in advance of BDC. It may also continue into the ensuing exhaust upstroke of the piston until the pressure drops to an extent insufficient to induce continued exhaust flow or until the upstroking piston has reduced the swept volume sufficiently to create pressure that forces the exhaust gases out through the open exhaust valve. Testing has shown that retarding the timing of exhaust valve opening can create more effective exhaust blow-down that is beneficial to turbocharger operation, particularly at low engine speeds where a turbocharger may have heretofore been considered relatively ineffective in improving engine performance.
Because certain principles of the present invention include changing the time in the engine operating cycle when the exhaust valves open, the engine must have an appropriate variable valve actuation mechanism for the exhaust valves. An example of such a mechanism comprises an electric actuator for opening and closing each exhaust valve in accordance with an electric signal applied to the actuator. Such an engine is sometimes referred to as a camless engine, particularly where the timing of the engine intake valves is also controlled by electric actuators. When the inventive strategy is invoked, the timing of the opening of each exhaust valve during the engine cycle is increasingly retarded.
By retarding exhaust valve opening, the in-cylinder burning time for particulates is increased, and this reduces particulate emission. Retarding the exhaust valve opening has also been discovered to provide increased energy input to the turbocharger compressor, thereby increasing boost, and it is believed that this discovery is a departure from presently prevailing knowledge. As boost increases and smoke decreases, engine fueling is also increased to develop increased engine torque so that the additional fueling is not adverse to tailpipe emissions in any significant way. In this way, the turbocharger is forced toward operating at its performance limit, thereby enabling the engine to develop a corresponding torque that is greater than the torque that would otherwise be achieved.
The consequence of retarding exhaust valve opening in conjunction with increasing engine fueling may however affect turbocharger operation. One possible consequence is undesirable surging of the turbocharger compressor that may occur should the exhaust flow acting on the turbine force the turbocharger to operate beyond its performance limit. In order to avoid such surging, a bleed valve at the compressor outlet operates at, or in anticipation of, incipient compressor surging to bleed compressed charge air from the intake system sufficiently to counteract, or prevent, the incipient surging. The compressed charge air is bled from the intake system in a manner that allows intake manifold pressure to increase without turbocharger surging. Because of this ability to achieve increased intake manifold pressure without accompanying turbocharger surging, the turbocharger is enabled to operate at or near its performance limit, even during low-speed engine operation, and in addition, the basic construction of the turbocharger, which is typically designed with high speed, rather than low speed, operation in mind, does not have to be modified or altered in order to implement principles of the present invention in an engine. Co-pending patent application Ser. No. 09/906,487, filed Jul. 16, 2001, discloses an electric-operated bleed valve associated with the engine intake system for bleeding compressed charge air from the intake system to counteract incipient compressor surging.
The invention of that the co-pending patent application relates to a strategy for controlling exhaust valve opening in a turbo-diesel engine that has a variable valve actuation mechanism. When the engine is running at less than peak torque speed, the engine control system causes the exhaust valves to open at a later time during the engine cycle than they would in an engine that has a camshaft operating the exhaust valves. The extent to which the control system retards exhaust valve opening is a function of one or more selected variables, such as engine speed, engine load, boost, brake specific fuel consumption (BSFC), and vehicle acceleration. The strategy is implemented via a processor-based engine control and utilizes data relating to certain engine operating parameters to control the bleed of compressed charge air from the engine intake system. The data is processed according to a software algorithm that is executed by the processor to develop data for a control signal that is applied to the bleed valve at the outlet of the compressor of the turbocharger. The controlled bleeding counters any incipient surging of the turbocharger resulting from increasingly retarding the timing of exhaust valve opening and accompanying increased fueling.
In one respect the present invention relates to a further improvement in that strategy that enables the bleed valve to be eliminated. Rather than adding a bleed valve for suppressing compressor surging, air is allowed to pass through the engine itself by operating the variable valve actuation mechanism to cause the exhaust valves to be open during intake portions of the engine cycle when the intake valves are open. The open exhaust valves have a suitable amount of lift and are kept open for a suitable amount of time to counteract incipient compressor surging whenever incipient surging is detected.
One general aspect of the claimed invention relates to an internal combustion engine comprising an intake system through which charge air is delivered to an intake manifold of the engine, including a turbocharger that comprises a compressor operated by exhaust gases from the engine for creating compressed charge air that provides boost in the intake manifold. A control controls the opening of engine exhaust valves in relation to an engine operating cycle and the fueling of the engine in relation to the engine operating cycle. At times, the control increasingly retards exhaust valve opening in relation to the engine operating cycle to cause the turbocharger to increase intake manifold pressure, increases engine fueling in relation to the increased intake manifold pressure, and in response to any incipient surging of the compressor resulting from the effect on engine exhaust gases of such increasingly retarded exhaust valve opening and such increased engine fueling, causes the exhaust valves to be open, during at least some portion of the time that engine intake valves are open, for bleeding a sufficient amount of charge air through the combustion chambers to the exhaust system to counteract incipient compressor surging.
Another general aspect of the claimed invention relates a method for an engine as just described wherein at times, exhaust valve opening is increasingly retarded in relation to the engine operating cycle to cause the turbocharger to increase intake manifold pressure, engine fueling is increased in relation to the increased intake manifold pressure, and in response to any incipient surging of the compressor resulting from the effect on engine exhaust gases of such increasingly retarded exhaust valve opening and such increased engine fueling, causing the exhaust valves to be open, during at least some portion of the time that the intake valves are open, for bleeding a sufficient amount of charge air through the combustion chambers to the exhaust system to counteract incipient compressor surging.
Further aspects of the claimed invention relate to details of the control strategy embodied in a software algorithm in the engine control processor for accomplishing the method just described, and to details of the way in which the engine valves are controlled.
The foregoing, along with further aspects, features, and advantages of the invention, will be seen in this disclosure of a presently preferred embodiment of the invention depicting the best mode contemplated at this time for carrying out the invention. This specification includes drawings, briefly described below, and contains a detailed description that will make reference to those drawings.